Do you the manual as it sounds like the second unit needs registering to the primary.
On page 54...
To register an additional handset to a base unit (easy registration)The included handset and base unit are preregistered. After purchasing an additional handset, register it to the base unit. Ensure that the additional handset is switched on. If it is not on, press and hold {ih} for few seconds to turn the handset on.1 Lift the additional handset and press {ih} to put the handset in standby mode.2 Press and hold {x} on the base unit for about 3 seconds, until the registration tone sounds.3 Place the additional handset on the base unit. The registration tone continues to sound. With the handset still on the base unit, wait until a confirmation tone sounds and w stops flashing.Note:L If an error tone sounds, or if w is still flashing, register the handset manually (manual registration; page 55).L If all registered handsets start ringing in step 2, press {x} to stop. Start again from step 1.L Charge the batteries of your additional handset for about 7 hours before initial use.L This registration method cannot be used for handsets that have already been registered to a base unit. Register the handset manually (manual registration; page 55).
To register a handset to an additional base unit (manual registration)You can register a handset to a base unit manually using the following method.1 Press $ (middle soft key).2 Select Y, then press ,.3 Select “Registration”, then press , 2 times.4 Select a base unit number, then press ,.L This number is used by the handset as a reference only.5 Press and hold {x} on the base unit for about 3 seconds, until the registration tone sounds.L If all registered handsets start ringing,press {x} to stop, then repeat this step.L After pressing {x}, the rest of this procedure must be completed within 1 minute.6 Wait until “Enter Base PIN” is displayed, then enter “0000” (default base unit PIN), then press ,.L If you changed the PIN, enter it (page 34).L When the handset has been registered successfully, w will stop flashing. If keytones are turned on (page 30), a confirmation tone will be heard.Selecting a base unitWhen “Auto” is selected, the handset will automatically use any available base unit it is registered to. When a specific base unit is selected, the handset will make and receive calls using that base unit only. If the handset is out of range of that base unit, no calls can be made.1 Press $ (middle soft key).2 Select Y, then press ,.Multi-unit Operation3 Select “Select Base”, then press ,.4 Select the desired base unit number, or “Auto”, then press ,.L The handset starts searching for thebase unit.